Sensor for measuring the concentration of a gaseous component in a fluid by absorption

Abstract

Apparatus and methods are provided for determining the concentration of a gaseous component in a fluid. A solid body of a natural or synthetic high polymer, which is permeable to the gaseous component, is exposed to the fluid, the polymer is exposed to infrared radiation, and the infrared absorption by the gas in the polymer is measured. In the preferred embodiment, a sensor is provided for making in vivo measurements of the concentration of CO 2 in the blood. The sensor includes an optical fiber which is nonpermeable to CO 2 and substantially transparent at the CO 2 absorption wavelength range, and a solid body of polymeric material at the distal end of the fiber which is substantially transparent to the absorption wavelength range and permeable to CO 2 . An incident infrared signal is transmitted down the fiber, passes through the body, is reflected off the distal end of the body, and the intensity of the return signal is measured by a detector. The return signal is diminished in proportion to the concentration of the CO 2 in the polymeric body. The sensor is disposed within a catheter and is positionable within the narrow blood vessels of the body for continuous real time monitoring of the carbon dioxide concentration of the blood.

What is claimed is: 
     
       1. Analytical apparatus comprising: waveguide means defining an axially-elongated energy path having an inlet aperture receptive to an incident energy signal within a predetermined wavelength range and an emission aperture for emitting an emitted energy signal, said waveguide means allowing passage of and being substantially transparent to energy within eh predetermined wavelength range;   an axial segment of said waveguide means comprising a block of fixed axial length of a solid material which is substantially transparent to energy within the predetermined wavelength range and permeable to a gaseous component that has an energy absorption peak in the predetermined wavelength range, the remainder of said waveguide means being substantially impermeable to the gaseous component;   whererin, when said axial segment is exposed along said fixed length to a fluid containing the gaseous component; the gaseous component permeates said segment and absorbs the incident energy signal along said fixed length to thereby reduce the intensity of the emitted energy signal in proportion to the concentration of the gaseous component in the segment, whereby differnces between the intensities of the incident and emitted energy signals may provide an indication of the concentration of the gaseous component in the fluid.   
     
     
       2. The apparatus of claim 1, wherein said waveguide means comprises an optical fiber having a distal end with said block disposed along its axial length.   
     
     
       3. The apparatus of claim 2, wherein said block has first and second ends, wherein said first end is disposed at the distal end of said optical fiber and said second end contacts a reflective surface.   
     
     
       4. The apparatus of claim 1, wherein said block comprises a solid polymeric material.   
     
     
       5. The apparatus of claim 4 for measuring the concentration of carbon dioxide in a fluid, wherein said optical fiber is made of a fluoride glass and said block is made of silicone and said fiber and block are substantially transparent to a predetermined wavelength range of from about 4.1 to about 4.4 micrometers.   
     
     
       6. Analytical apparatus comprising: a catheter body having a distal end positionable in vivo,   waveguide means carried by the catheter body and defining an axially-elongated energy path having an inlet aperture for receiving an incident energy signal within a predetermined wavelength range and an emission aperture for emitting an emitted energy signal, said waveguide means allowing passage of and being substantially transparent to energy within the predetermined wavelength range;   an axial segment of said waveguide means comprising a block of fixed axial length of a solid material which is substantially transparent to energy within the predetermined wavelength range and permeable to a gaseous component that has an energy absorption peak in the predetermined wavelength range, the remainder of said waveguide means being substantially impermeable to the gaseous component;   wherein, when said axial segment is exposed along said fixed length to a body fluid containing the gaseous component, the gaseous component permeates said segment and absorbs the incident energy signal along said fixed length to thereby reduce the intensity of the emitted energy signal in proportion to the concentration of the gaseous component in the segment, whereby differences between the intensities of the incident and emitted energy signals may provide an indication of the concentration of the gaseous component in the patient's body fluid.   
     
     
       7. The analytical apparatus of claim 6, wherein the waveguide means comprises an optical fiber having a distal end with said block disposed along its axial length.   
     
     
       8. the analytical apparatus of claim 7, wherein said block has first and second ends, wherein said first end is disposed at the distal end of the fiber and said second end contains a reflective surface.   
     
     
       9. The analytical apparatus of claim 8 for measuring the concentration of carbon dioxide in the blood, wherein said optical fiber is made of a fluoride glass and said block is made of silicone and said fiber and block are substantially transparent to a predetermined wavelength range of from about 4.1 to about 4.4 micrometers.   
     
     
       10. The analytical apparatus of claim 6, wherein said catherter has an aperture adjacent its distal end and said block is disposed adjacent said aperture so as to allow contact with body fluid.   
     
     
       11. A system for measuring the concentration of a gaseous component in a fluid by absorption, the gaseous component having an energy absorption peak lying within a predetermined wavelength range, said system comprising: waveguide means defining an axially-elongated energy path having an inlet aperture receptive to an incident energy signal within the predetermined wavelength range and an emission aperture for emitting an emitted energy signal, said waveguide means for allowing passage of and being substantially transparent to energy within the predetermined wavelength range;   an axial segment of said waveguide means comprising a block of fixed axial length of a solid material which is substantially transparent to energy within the predetermined wavelength range and permeable to a gaseous component to be measured, the remainder of said waveguide means being substantially impermeable to the gaseous component;   wherein, when said axial segment is exposed along said fixed length to a fluid the gaseous component to be measured will permeate said segment and absorb an incident energy signal along said fixed length to thereby reduce the intensity of an emitted energy signal in proportion to the concentration of the gaseous component in the segment;   energy source means for directing to said inlet aperture the incident energy signal;   a detection means and means for directing the emitted energy signal from said emission aperture to said detection means, said detection means being constructed and arranged to provide an indication of the concentration of the gaseous component in the fluid as a function of the intensity of the emitted every signal.   
     
     
       12. A fiber optic sensor for measuring the concentration of a gaseous component in a fluid by absorption, the gaseous component having an energy absorption peak lying within a predetermined wavelength range, said sensor comprising: an optical fiber which is substantially impermeable to the gaseous component and substantially transparent to the predetermined wavelength range, said fiber being axially-elongated and having a proximal end and a distal end, the proximal end being coupled to means for inputting an incident energy signal within the predetermined wavelength range and being further arranged to emit an emitted energy signal;   a block of fixed axial length disposed at the distal end of said fiber, said block being made of a solid material which is substantially transparent to the predetermined wavelength range and permeable to the gaseous component, said block having a first end coupled to said distal end of said fiber and a second end having an inwardly reflective surface, wherein, when said block is exposed to the fluid along said fixed length the gaseous component permeates said block and absorbs the incident energy signal as it travels along said fixed length to thereby reduce the intensity of the emitted energy signal in proportion to the concentration of the gaseous component in the block, and   means for detecting the emitted signal for measuring differences between the intensities of the incident and emitted signals and for providing an indication of the concentration of the gaseous component in the fluid based on said differences.   
     
     
       13. The fiber optic sensor of claim 12 for measuring the concentration of carbon dioxide in a fluid wherein said optical fiber and block are substantially transparent to a predetermined wavelength range which corresponds to an absorption peak for carbon dioxide.   
     
     
       14. The fiber optic sensor of claim 13, wherein said fiber and block are substantially transparent to a predetermined wavelength range of from about 4.1 to about 4.4 micrometers.   
     
     
       15. The fiber optic sensor of claim 14, wherein said fiber is made of a material selected from the group consisting of fluoride glass, chalcogenide glass, chloride glass, silver halide, and potassium halide; and   said block is made of a material selected from the group consisting of silicone, polystyrene, polyurethane, polyethylene, cellulose, polybutadiene, poly(methylmethacrylate), and polycarbonate.   
     
     
       16. The fiber optic sensor of claim 15, wherein said fiber is a heavy metal fluoride glass and said block is silicone.   
     
     
       17. The fiber optic sensor of claim 16 for measuring a partial pressure of carbon dioxide of from about 10 to about 100 mm Hg, wherein said block has a length of from about 0.5 to about 2 mm.   
     
     
       18. The fiber optic sensor of claim 17, wherein said block has a length of about 1 mm.   
     
     
       19. The fiber optic sensor of claim 12, wherein said second end of said block is covered with a reflective metal coating.   
     
     
       20. The fiber optic sensor of claim 12, wherein substantially the entire outer surface of said block, other than said first end, is at least partially covered by a reflective metal coating.   
     
     
       21. The fiber optic sensor of claim 12, wherein said block comprises a cylinder coaxially adhered to the distal end of the fiber.   
     
     
       22. The fiber optic sensor of claim 12, wherein said block comprises a cylinder of silicone coaxially adhered to the distal end of the fiber.   
     
     
       23. The fiber optic sensor of claim 12, wherein said fiber and block are disposed within a flexible catheter adapted for insertion in a body cavity for making an in vivo determination of the concentration of a gaseous component in a body fluid.   
     
     
       24. The fiber optic sensor of claim 12, wherein said fiber and block are disposed within a flexible catheter adapted for insertion in the bloodstream of a patient for making an in vivo determination of the concentration of a gaseous component in the blood.   
     
     
       25. A method for detecting a gaseous component in a fluid by absorption, the gaseous component having an energy absorption peak lying within a predetermined wavelength range, said method comprising the steps of: directing an incident energy signal within said predetermined wavelength range to an inlet aperture of an axial waveguide means, the waveguide means allowing passage of and being substantially transparent to energy in the predetermined wavelength range and being substantially impermeable to the gaseous component except for a fixed-length axial segment thereof comprising a block of solid material which is permeable to the gaseous component;   exposing said axial segment along the fixed length thereof to a fluid containing the gaseous component, wherein the gaseous component permeates the segment;   passing the incident energy signal through said waveguide means, wherein the incident energy signal is absorbed by the gaseous component along the fixed length of the axial segment to thereby reduce the intensity of the emitted energy signal in proportion to the concentration of the gaseous component in the segment;   detecting the emitted energy signal from an emission aperture of said waveguide means; and determining the concentration of the gaseous component from the detected emitted energy signal.   
     
     
       26. The method of claim 25 for measuring the concentration of a gase having an energy absorption peak in the infrared region, wherein an incident energy signal having a wavelength in the infrared region is directed to the inlet aperture.   
     
     
       27. The method of claim 26 wherein said gaseous component is selected from the group consisting of carbon dioxide, water vapor, nitrous oxide, halogenated hydrocarbon, ethyl alcohol, and anesthetic gases, and wherein the incident energy signal is selected to have a wavelength corresponding to an absorption peak of the selected gas.   
     
     
       28. The method of claim 27, for the measurement of the carbon dioxide concentration in blood, wherein an incident energy signal having a wavelength of from about 4.1 to about 4.4 micrometers is directed to the inlet aperture.